The present invention relates to a method for the preparation of a positively temperature-dependent organic resistor, i.e. an organic resistor material having positive temperature-dependency of the resistivity. More particularly, the invention relates to a method for the preparation of a positively temperature-dependent organic resistor by effecting a crosslinking reaction in a blend of a crystalline synthetic resin and electroconductive particles by using a specific crosslinking agent.
It is known that an organic resistor prepared by dispersing electroconductive particles in the matrix of a crystalline synthetic resin has positive temperature dependency of the resistivity. Positively temperature-dependent organic resistors of this type generally have a problem that melting of the matrix resin may take place when the resistor is used at a relatively high temperature or the matrix resin is subject to degradation in the lapse of time. The problem due to melting of the matrix resin can be solved by crosslinking the resin so that the positive temperature coefficient of the resistivity can be maintained even at a temperature higher than the melting point of the uncrosslinked matrix resin. Crosslinking of a synthetic resin is carried out usually by the method of irradiation with electron beams or by the chemical method using an organic peroxide as a crosslinking agent (see, for example, U.S. Pat. No. 3,351,882).
These conventional methods, however, have respective disadvantages. For example, the crosslinking method by electron beam irradiation has a problem that the resistivity of the resistor prepared by this method is subject to a gradual change when the resistor is used repeatedly in addition to the economical disadvantage due to the very high costs for the facilities to conduct electron beam irradiation. The chemical crosslinking method is also defective because, in addition to the problem that the resistivity of the resistor prepared by the method is also subject to gradual changes by the repeated use of the resistor, the crosslinking reaction may proceed already in the blending step of the crystalline synthetic resin and the electroconductive particles affecting the uniformity of dispersion of the particles in the matrix so that the resistor prepared by this method sometimes has an increased resistance unless the volume fraction of the electroconductive particles is increased so much.